In tissue regeneration strategies, 3-D scaffolds prepared from collagenous foams or sponges have been used to help restore damaged or missing tissues, or correct tissue voids. Natural collagenous foams are of interest because they provide the initial extracellular matrix (ECM) foundation for cells to attach and proliferate. A variety of scaffolding biomaterials derived from collagen are available. Porosity of these materials is critical since it allows for cellular penetration, nutrient and oxygen diffusion, and has been shown to direct the cell response in terms of viability, proliferation, migration, and differentiation by mediating 2-D cell spreading and 3-D intercellular contacts, depending on the pore size.
Foams have been fabricated with purified collagen from calf skin, bovine collagen, gelatin, porcine fetal collagen, and purified collagen type I (porcine, bovine, rat). Generally, the processes for fabricating these foams involve solubilization of the collagenous material followed by drying to yield a porous structure. In the solubilization step, a dilute acid is often used with or without an additional enzymatic digestion to create a collagen suspension that would otherwise be insoluble in aqueous solution. The solution is then poured into a preformed mould where it is frozen and freeze-dried, or in some cases immersed in ethanol and critically point dried. This general approach is dependent on ice crystal formation as a porogen and can be easily controlled by varying the collagen solution concentration and freezing temperatures. Other variations on this method include solvent-casting, emulsion freeze drying, particulate leaching, and gas foaming.
In vivo studies have shown that foams fabricated in this fashion exhibit poor cellular infiltration with only a few cells migrating as far as 500 μm into the foams, alone with an observable monolayer growth of up to approximately 100 μm. A major challenge arises in poor diffusion of nutrients and oxygen into the interior, as the surface pores are blocked by the expansion of cells over time. As a result, several groups have addressed the need for a long ranging channeling microarchitecture construct. In particular, the use of solid free form (SFF) technology is gaining popularity in which 3-D printers are used to fabricate custom casting moulds designed using computer-aided design (CAD) software. For example, using available layer-by-layer 3-D printing techniques, complex channels larger than 100 μm can be achieved with high degree of control and resolution. However, drawbacks include the inherent difficulty in removing residual powders as well as toxic solvents and binders in the complex channels, poor mechanical strength of the constructs, and in some cases high temperatures are used which can degrade biological components. In addition, the use of sacrificial moulds has also been explored whereby moulds constructed using SFF are filled with a collagen solution. Upon solidifying the collagen solution, the mould is degraded thermally or chemically, but once again, residual moulding materials and extreme techniques may prove unfavourable to the final product.
An alternative approach in forming porous foams uses an ice particulate template method whereby pore size can be made larger at the surface than the interior. This strategy depends solely on ice crystal formation to control the degree of porosity. Ice particles are formed by spraying water onto a plate and frozen at various temperatures to achieve different sized spheres. The ice particulates are embossed onto a silicone frame into which a solution of supercooled collagen is poured. Following this, the entire construct is frozen and lyophilized. Since many of these foams have poor mechanical strength upon fabrication, crosslinking agents such as glutaraldehyde. EDC/NHS, and genipin have been used. However, crosslinking presents cytotoxicity risks and may affect the porosity of the foam, as it is difficult to control the degree of crosslinking desired.
Commercially-available foams approved for clinical use include Colla Plug®, Colla Cote®, and Colla Tape® (Zimmer Dental Inc., U.S.A.) a family of resorbable bovine collagen type I plugs, foams, and tapes. In addition, Gelfix® (Abdi ibrahim, Turkey), a foam prepared from lyophilized collagen, and GelFoam® (Pfizer), a sterile sponge prepared from porcine skin gelatin USP granules, are also used in surgical procedures. Although such materials are easily accessible and acceptable for human use, these products may pose xenogenic risks.
Clearly there is a need for a foam material without the above drawbacks for use in wound healing, soft tissue regeneration and augmentation applications.